In general, a spark plug used for igniting an internal combustion engine such an automotive engine includes a tubular metallic shell; a tubular insulator disposed in the bore of the metallic shell; a center electrode disposed in a forward end portion of the axial hole of the insulator; a metallic terminal disposed in a rear end portion of the axial hole; and a ground electrode whose one end is joined to the forward end of the metallic shell and whose other end faces the center electrode so as to form a spark discharge gap.
The center electrode has a leg portion disposed in the forward end portion of the axial hole, and a large-diameter portion located rearward of the leg portion and having a diameter grater than that of the leg portion. The center electrode is disposed such that the large-diameter portion is supported by a ledge portion of the insulator at which the diameter of the axial hole changes. In some cases, the center electrode has a projection portion which is located rearward of the large diameter portion and which has a diameter smaller than that of the large-diameter portion. A seal is provided around the large-diameter portion and the projection portion; i.e., in the space between the insulator and the large-diameter portion and projection portion of the center electrode. This seal fixes the center electrode within the axial hole.
Patent Document 1 discloses a spark plug in which charging of the seal is improved so as to enhance the shock resistance of the center electrode to a sufficient degree; specifically, a “spark plug which satisfies relational expressions 13≦B/A≦40 and 10≦C/A≦35, where B represents the distance between the peripheral edge of the center electrode and an end of a parallel groove formed on a head portion of the center electrode and crossing the head portion in the diametrical direction, and C represents the height of a projection which is formed on the head portion of the center electrode as a result of formation of the parallel groove” (see claim 1 of Japanese Patent Application Laid-Open (kokai) No. H9-266055).
In order to fix the center electrode within the axial hole in a good condition, it is desired that the seal be uniformly provided around the center electrode. However, a problem arises when the shape of the projection portion of the center electrode is not axisymmetric with respect to the axis of the center electrode; for example, when the axis of the circular columnar projection portion deviates from the axis of the center electrode in the radial direction. Specifically, since the space between the center electrode and the insulator is not formed uniformly in the circumferential direction, the seal may have a thick portion and a thin portion. As a result, when a resultant spark plug is used on an actual engine, because of vibration, thermal expansion caused by high temperature, or other causes, the center electrode may rattle within the axial hole, while the thin portion of the seal serves as a weak point.